Add support for combining the notices from 2 roots.

Apparently needed for cts tests due to apks putting intermediate files under
TARGET_OUT even when going to HOST_OUT.

Test: m with and without change verified no differences in notice files
Bug: 141258651

Change-Id: Iea50b6bd9af9452a2600c97e2f7cb89218b9be7f
(cherry picked from commit 5e9e1fbe)
Merged-In: Iea50b6bd9af9452a2600c97e2f7cb89218b9be7f